Magnetic inks form an essential feature of jet printing. An ingredient employed to make ink is magnetite or Fe.sub.3 O.sub.4 which is black in color and is magnetic. When small particles of such Fe.sub.3 O.sub.4 are used as an ingredient of printing ink, printing can occur by causing the magnetic ink to be emitted as a narrow jet from a suitable source. While in flight, the magnetic ink is deflected by permanent magnets or electromagnets onto a recording surface moving relative to the jet stream. Many articles concerning ink jet printing have been published and only a few will be cited herein merely to give background material of the technology.
"Viscosity of Magnetic Fluid in a Magnetic Field" -- R. E. Rosenzweig et al. -- Journal of Colloidal and Interface Science (29) pp. 680-686 (1969).
"Ink Jet Printing" -- J. Stone -- The Electronic Engineer, December 1971.
"Magnetic Fluids" -- R. Rosenzweig -- International Science and Technology, pp. 48-56, July 1966.
Magnetite is especially suitable for making magnetic inks because (1) it is naturally black (2) it can be made small, i.e., .about.50 to 200 A and (3) has a high magnetic moment, for example, about 100.sup.+ emu/gm. One very popular way to achieve small particles of Fe.sub.3 O.sub.4 suitable for making ink was to buy magnetite commercially and use ball milling machines to crush large particles into small particles. Such mechanical milling is not only very time consuming, but obtaining particles less than 100 A by such method is difficult.
Applicants are also aware that chemical techniques may be employed whereby Fe.sub.2 O.sub.3 is precipitated from a solution and by an oxidation process Fe.sub.2 O.sub.3 is converted into Fe.sub.3 O.sub.4. Other chemical precipitation techniques are illustrated in two U.S. Pat. Nos. 3,228,881 and 3,228,882, both of which were filed on Jan. 4, 1963 and issued on June 11, 1966.
The method that forms the present invention relies on an evaporation technique wherein pure iron Fe.degree. which has a high magnetic moment, is formed, surrounded by the oxide Fe.sub.3 O.sub.4. The evaporation method described hereinbelow produces Fe.degree. particles each of which oxidizes by about 30% of its volume, after which particle growth stops for all practical purposes. Since Fe.degree. has a high magnetic moment, a particle that is mainly pure Fe.degree. and is black lends itself to use as an ink component.